Noisemaker Workshop Ring Modulation and Oscillator Sync (and more!) May 14, 2009 Outline Ring Modulation Sync and Pulse-width Modulation Outline Ring Modulation Sync and Pulse-width Modulation Real Ring Modulation What is it? I Combine two signals into one by multiplying their voltages I Ring modulation creates a signal with the sum and the difference of the input signals I Stripped-down example: Two sine waves, 100 Hz and 600 Hz I Ring-modulating them results in a two tones superimposed: one at 500Hz (the difference) and one at 700 Hz (the sum) I Neat property: if you ring-modulate a signal with another (carrier) then ring-modulate the result with the carrier again, you get the original signal back I Cool use: carrier is a noisy stream, can use to encode audio I Pedestrian use: radio transmitters ring-modulate the audio signal with a RF carrier, receivers ring-modulate with the carrier to get audio back (tuning) XOR "Ring Modulation" XOR Logic I The XOR ("exclusive or") function gives an output that's high when one or the other oscillator is high, but not both I Logic Table: 0 0 ! 0 1 0 ! 1 0 1 ! 1 1 1 ! 0 I XOR'ing makes new frequencies like true ring modulation. Sometimes they're harmonically related, sometimes not. I XOR'ing also has the encode/decode property of real ring modulation, but for digital signals XOR Ring Modulation Circuit Outline Ring Modulation Sync and Pulse-width Modulation Sync Saw this before? I Use a diode (valve) to reset one oscillator from the output of another I Fun noises when you change the sync'ing frequency I I mention this again, because it's fun with ring modulation Pulse-Width Creating non-symmetric oscillations I To create a pulse, connect output of one oscillator to the input of the other with a capacitor I The capacitor passes a small bit of current at the start of the pulse, but none of the low-frequency stuff I Then use a resistor to keep the input high most of the time I How quickly the input goes high again (after a low pulse through the cap) depends on the value of a resistor that pulls it high I Pulse-width modulation by changing resistor makes phaser sounds Sync Sync with Multiple Oscillators More fun I If you're syncing a bunch of oscillators, makes a neat sound I Plus, it's sweet to ring-modulate the results I Make the sync pulse very thin, nearly instant I Take the pulse out (cyan line in the above circuit), link it in to other oscillators with a bunch of diodes. Diode Mixing As long as we're talking about diodes... I Run output of two oscillators together into same output, it's just like playing them through two speakers I Instead, combine them with two diodes, and they add together but never subtract. The End Outline.